Respiration systems are usually used for medical applications to support breathing and/or to administer anesthetics during surgical procedures. While the breathing gas circulates in a closed circuit in anesthesia systems and is fed again to the patient after the carbon dioxide contained in the expired air has been removed in a carbon dioxide absorber, an open circuit is usually used in respiration systems in intensive care, i.e., the expired air is fed to the room air. Bacteria filters are used to maintain the breathing gas flowing in respiration systems germ-free. Furthermore, devices for enriching the breathing gas with moisture may be present in respiration systems in order to bring about moistening of the patient's airways. Bacteria filters may be arranged on both the inspiration side and the expiration side of the breathing circuit.
Condensation may occur due to moisture present in the breathing gas because of active moistening and/or due to the moisture present due to expiration by a patient. Condensation in the bacteria filter leads to moistening of the filter material and it brings about, as a result, a reduction of the permeability to air of the filter over time, because the filter resistance increases. This compromises the patient's ability to breath and leads to the need to replace bacteria filters more often than it would be necessary based on contamination by bacteria alone.
An electric heating element, by means of which the filter is heated, is used, for example in U.S. Pat. No. 4,727,871 to prevent condensation in an antibacterial filter. The heating elements used for this and the means necessary for operation for controlling and for supplying the heating elements with power make the manufacture of such a filter complicated and expensive.
Instead of an active heating of a filter or of a filter housing surrounding the filter, another housing is arranged around the filter housing in US 2001/0029949A1, so that an air gap is formed between the filter housing and the additional housing. The condensation within the filter housing shall be reduced by the insulating action of the air gap. However, the insulating action of such an air gap is limited, so that long-term use for more than one day is not possible with such filters.